Palos Verdes Shelf Experiment: What Will Happen to the Contaminated Mud?

Let's say there's a pile of mud that's been accumulating on a continental shelf since the Marx Brothers made A Day at the Races in 1937. Then let's turn off the main source of sediment for that deposit. Will it erode?

This is the deceptively simple question that U.S. Geological Survey (USGS) scientists must answer at the Superfund site on the shelf off the Palos Verdes Peninsula in southern California. The pile of mud is an effluent-affected deposit containing material discharged from the Los Angeles County Sanitation District's outfalls off Whites Point.

In addition to sewage associated with fine sediment, the outfalls once discharged DDT, PCBs, and other pollutants. In 1971, the DDT-manufacturing plant was disconnected from the treatment system, and in the following years the effluent stream and discharged sediment became progressively less contaminated. By 2005, all of the effluent was receiving advanced secondary treatment, and the discharged fluid is now almost devoid of sediment; it has an average solids concentration of 17 mg/L, not all of which sinks.

The sedimentary deposit off the Palos Verdes Peninsula now has two layers: a bottom layer that contains material deposited before 1970 and is contaminated with some of the highest levels of DDT measured in open marine environments (more than 250 ppm), and an overlying, cleaner layer that insulates the bottom layer from the pelagic environment. The main part of the deposit lies along the 60-m isobath and is less than 1 m thick; the relatively clean upper layer is 20 to 30 cm thick. All of this sediment is stiff, gray, silty to sandy mud, and, except for a fluffy layer on top, it is difficult to erode. The question we hope to answer is: Will the cleaner upper layer continue to sequester the pollutants, or will it gradually erode and eventually allow the release of buried DDT and PCBs?

Above: Study area on the Palos Verdes shelf encompassing 6 sites (B1-B7), with 13 platforms (yellow diamonds) housing oceanographic instrumentation. Three of the sites had been occupied in 2004, one of which was a Geoprobe site in 1992. Sea-floor image from USGS multibeam bathymetry; aerial photographs from the California Spatial Information Library. Light-orange line, outline of sea-floor area with higher DDT concentrations; red-brown lines, Los Angeles County Sanitation District outfall pipes. Ocean-floor contours in 5-m increments from 5- to 100-m depth and in 100-m increments below 100-m depth. National Data Buoy Center station 46222 is just below bottom of map, about 8 km south of station B5. [larger version]

The USGS has been involved in Palos Verdes shelf studies since 1990, when the U.S. Department of Justice, plus the National Oceanic and Atmospheric Administration (NOAA) and the other Natural Resource Trustees, asked USGS scientists to determine the fate of the contaminated deposit and enlisted them as experts in a Federal suit against the manufacturers of the DDT. The legal battle led to some excellent science on both sides. The USGS mapped the Palos Verdes shelf in 1992; conducted measurements of waves, currents, and sediment transport; and modeled the evolution of the deposit. Scientists hired by the defendants made ground-breaking measurements of DDT degradation, proving that DDE (a degradation product of DDT and the most common DDT-related component) can lose chlorine during in-place transformation to DDMU in Palos Verdes shelf sediment. (DDMU is another breakdown product that may pose a lesser risk of accumulation in the food chain.) In the end, the plaintiffs won a consent decree in 2000, and a settlement of $136 million was divided among the Trustees and the U.S. Environmental Protection Agency (EPA) to be put toward a remedy. In the meantime, the Palos Verdes shelf off Whites Point was placed on the Superfund National Priorities List, and the EPA is now responsible for determining whether to try to somehow clean up or cap the deposit, or monitor it and spend remediation money elsewhere.

Although the legal issues were settled, the fundamental question concerning the fate of the DDT was left unanswered. Earlier studies found that bioturbation (mixing of sediment by mollusks, worms, and shrimp) and wave-induced sediment resuspension (lifting of sediment back into the water) were key to the fate of sediment and DDT on the Palos Verdes shelf (see Sound Waves, July 2002, "Contaminated Sediment Off Palos Verdes, CA, the Subject of a Special Issue of Continental Shelf Research"). DDT was being (1) mined from deeper sediments by burrowing fauna, (2) resuspended by strong wave events, (3) desorbed from sediment during resuspension events, (4) transported from the shelf with sediment, and (5) transformed in place. Measurements indicated that waves and currents were uniform over much of the shelf, and erosion and deposition patterns were believed to depend on subtle changes in bed sediment. In particular, we worried that the southeast edge of the deposit was eroding.

Above left: Waves break south of Whites Point and resuspend sediment as the giant swells of late February subside. Photograph by Chris Sherwood. [larger version]Above right: Plastic sheets that slowly absorbed PCBs from surrounding water were deployed during the Palos Verdes experiment for collaborators from the EPA and University of Rhode Island. Photograph by Chris Sherwood. [larger version]

The EPA provided funding for fieldwork by Science Applications International Corporation (SAIC), the USGS, and others in 2004 to map the geotechnical properties of the deposit, evaluate erodibility and bioturbation, and conduct measurements in the bottom boundary layer (the layer of water just above the sea floor that is particularly important for moving sediment). In addition, USGS scientists Marlene Noble, Jingping Xu, and Kurt Rosenberger analyzed a valuable series of current-meter measurements (now extending to nearly 6 years at 13 sites on the Palos Verdes and San Pedro shelves) made by the Los Angeles County Sanitation Districts. These analyses indicate that internal bores (solitary waves traveling on density interfaces) associated with internal tides cause persistent near-bottom flows that are sometimes strong enough to erode and transport sediment, but our understanding of what controls the timing and distribution of these events is sketchy. The bottom-boundary-layer measurements from the 2004 field program were disappointing: it was a very calm year, and only one tepid sediment-resuspension event occurred while the instruments were in the water.

This past winter (2007-08), the EPA supported a more ambitious program to measure bottom-boundary-layer processes that affect transport of sediment and contaminants on the Palos Verdes shelf. The objectives of this field program were to measure (1) bottom stresses and suspended-sediment concentrations in order to determine thresholds for erosion and the transport rates for sediment along the 60-m isobath (with instrumented tripods that sit on the seabed), (2) internal-wave motions at several alongshore and cross-shelf sites (with moored temperature arrays and current profilers), and (3) temporal changes in sediment erodibility (with erosion-chamber measurements).

Above: Waves during the Palos Verdes experiment. Top panel, significant wave height (Hs) at the San Pedro Channel Waverider Buoy (National Data Buoy Center station 46222); middle panel, peak wave periods (Tp) measured at the buoy (blue) and calculated for 60-m depth (red); bottom panel, near-bottom wave-orbital velocity (ub) at 60-m depth, calculated from buoy data. Dashed line, approximate threshold for resuspension of Palos Verdes sediment (0.14 m/s). The following times are indicated by arrows in the top panel: Deployment of all instruments in December, early release of the subsurface float at site B3, recovery of most instruments in late April, and recovery of last instrument at site B1 in early May. [larger version]

The instruments were deployed in early December 2007 from the Scripps Institution of Oceanography's research vessel Robert Gordon Sproul. A total of 13 moorings were deployed at six sites, supporting 65 instruments with data loggers recording an uncounted number of individual sensors. The somewhat lengthy saga of recovering all of these instruments began in February 2008 and was completed in early May (see "Palos Verdes Shelf Program: Whatever Can Go Wrong…," this issue).

During the deployment cruise, Professor Pat Wiberg (University of Virginia) made erodibility measurements on the (nearly) pristine tops of cores obtained with Mike Bothner'shydraulically damped gravity corer. Those measurements, which were repeated in February and May, may reveal temporal changes in erodibility.

In addition to all the fancy electronics, we also deployed passive PCB samplers for researchers Robert Burgess (EPA's Atlantic Ecology Division Laboratory in Narragansett, Rhode Island) and Rainer Lohmann (University of Rhode Island). To the untrained eye, the PCB samplers look like little sheets of plastic. They are. But their ability to absorb hydrophobic contaminants like PCBs has been carefully characterized, and attached to various parts of our moorings, they acted as long-term samplers.

Above left: Rigging the Geoprobe tripod for deployment on the Palos Verdes shelf on a calm day in early December 2007. Photograph by Marinna Martini. [larger version]Above right:Marlene Noble and Chris Sherwood ponder their notes during the Palos Verdes mooring recovery in May. Photograph by Brandy Armstrong. [larger version]

Luckily for the experiment, 2007-08 was an eventful winter, as most Californians can attest. Three of the largest wave events we have measured at the seabed off the Palos Verdes Peninsula occurred during the deployment, and preliminary examination of the data shows significant resuspension and transport of sediment. We hope this experiment helps us understand the role of internal waves and provides data that will constrain our sediment-transport models and ultimately help us answer the question: Will the mud move or stay there?